The known machines of this type generally include a load-bearing structure supporting a pair of motorized rollers which are placed one above the other. The upper roller deposits the dye or adhesive, the lower roller is used to feed the sheets of leather forward beneath the dye roller. Upstream of the rollers there are provided a pair of reciprocally facing conveyor belts, which rotate in the same direction, and partly contact each other to forward the hides towards the impregnation area.
The two rollers have substantially parallel axes, and are spaced slightly apart to form a converging-diverging channel with a reduced section which is slightly less than the thickness of the treated hides. The upper roller has a pad which continually impinges its surface with the impregnating liquid, this liquid being transferred to the upper surface of the hides in the restricted section of the converging-diverging channel. The lower roller is in contact with the hides over a set contact range so it transmits to the hides tangential forces which are sufficient to overcome the resistance of the upper roller while liquid is being deposited. Both rollers are connected with an electric motor by means of drive belts or similar devices, and can respectively rotate in the same direction or in opposite directions.
In the first case, the impregnating roller which contacts and slightly compresses the upper surface of the hides exerts tangential forces on the hides favoring movement thereof while the liquid is being deposited, however it does not ensure that the liquid is deposited in a perfectly uniform fashion as it is continually repelled away at the reduced section of the channel.
In the second case, on the contrary, the liquid is somewhat "spread" over the contact area, and is evenly distributed over the entire surface of the hides yielding high quality results. However, the tangential forces exerted by the upper roller on the hides act in an opposite direction with respect to the driving forces of the lower roller, and can cause a local lifting which appears as a wrinkle, in the area immediately upstream of the reduced section, where the leather is not adequately supported. This "wrinkle" can progressively degenerate and increase forming an actual roll of material, commonly termed "cigarette". It is apparent that such an accumulation of material may cause the hides to be incompletely impregnated and, in some cases, it may cause irreparable damages if the machine is not immediately stopped.
This kind of drawback mainly occurs when very soft thin leather is treated because of its inability to counter the tangential force created by the impregnating roller.
Various expedients have been adopted in known machines in order to solve the preceding problem, such as the forward offset of the upper roller with respect to the lower roller so as to create a converging-diverging channel which is slightly sloped down so that forwarding of the leather is favoured by gravity. Another solution known in the art consists of placing the tensioning roller of a conveyor belt which follows the impregnating rollers as close as possible to the reduced section of the converging/diverging channel, so as to reduce as much as possible the length of the leather which is not guided.
There is also another configuration employing a conveyor belt where the belt travels through the converging channel created by the rollers and extends both before and after them, so as to considerably increase the drive surface in contact with the leather.
Another solution consists of covering the drive roller with a uniform layer of rubber, or a similar elastomer, so that the friction coefficient is increased and drive improved.
Although these expedients can reduce the risk of wrinkling of the hides which are more than 1 mm thick, they are no longer effective for hides with lower thickness, and thus, after the machine has been operating for a certain time, they are unable to prevent the hides from rolling and forming a "cigarette". It is consequently necessary to stop the machine, separate the rollers, and stretch the hides before the machine can be started again; this involves considerable waste of labor time, low productivity, and continual attendance of control personnel.